Which of the following is a key feature of the Australian electoral system?

The key feature of the Australian electoral system is the principle of compulsory voting. This means that citizens who are eligible to vote must participate in elections or face penalties. This system was introduced to ensure higher voter turnout and to reflect the will of the population more accurately in the electoral outcomes. It promotes civic responsibility and reduces the influence of political campaigning aimed solely at mobilizing a base of motivated voters, as the overall voting pool is maximized.

In contrast to this, optional voting would allow citizens to decide whether or not to vote, which could lead to lower participation rates and potentially skewed representation. First-past-the-post voting refers to a system where the candidate with the most votes wins, which is not how most Australian elections are conducted, especially for the House of Representatives where a preferential voting system is utilized. Proportional representation is used in the Senate, but it isn't the defining feature of the electoral system as a whole. Thus, compulsory voting is a distinct characteristic that underpins the integrity and inclusiveness of Australia's democratic processes.